Leif Erik,

When you indicate that you confirmed the failure due to silver sulfide do you mean that you can see silver migration or silver sulfide corrosion products shorting features or corrosion causing an open circuit?  I couldn't tell from your email if the exposed silver on the resistor network was corroding and causing an open circuit or if silver had migrated causing a short circuit.  It is not uncommon to see tarnish and or silver sulfide corrosion on products with exposed silver.  However, the only failures we've seen were primarily due to ionic residues left behind from assembly processes causing creep corrosion.  When residues aren't present we still see various degrees of silver sulfide on silver surfaces from a light tarnish to a heavy Brown/Black appearance depending on the amount of air pollution and exposure time but no migration or detrimental corrosion.

Regards,
George
George M. Wenger
Senior Principal Reliability / FMA Engineer
Andrew Corporation - Wireless Network Solutions
40 Technology Drive, Warren, NJ 07059
(908) 546-4531 Office (732) 309-8964 Mobile
E-mail: [log in to unmask]

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Leif Erik Laerum
Sent: Wednesday, May 25, 2011 9:12 AM
To: [log in to unmask]
Subject: [TN] Silver Sulfide contamination

  Technetters,

We have confirmed that a returned product from the field has failed die 
to silver sulfide contamination.

The conclusion from the analysis is:
Based on the analysis performed, the "black" contaminant material is 
silver sulfide, a common silver corrosion product. This corrosion often 
occurs when silver plating is exposed to atmospheric pollution that 
contains high levels of sulfur compounds (exhaust from fossil fuels).

We have confirmed that in particular resistor networks are susceptible 
due to the silver content that apparently is exposed on the top of the 
component. We have not observed issues with any other component types.

The obvious answer to how to protect a product in this environment is to 
use conformal coating. However, from what I have read there are only 
very few coating materials that can actually protect from silver sulfide 
corrosion. Epoxy based conformal coatings is the only ones that will 
successfully pass ASTM N 809 test. My question to the experts is if 
there are other solutions or coatings you have been successful with or 
if someone has actually solved this issue in different ways. It seems 
this should be an important issue for the automotive electronics industry.

Some one suggested using solder to cover the component, but I can not 
understand how that could work.

As always, thank you for your insight.

-- 
Leif Erik Laerum




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